Interleukin 10 (IL-10) is a cytokine which is intimately involved in the regulation of immune and inflammatory responses. Several studies have documented its essential role in preventing prolonged and exaggerated immune responses to antigens and irritants. The safety, tolerance, immunologic effects and clinical activity of multiple doses of subcutaneous recombinant human IL-10 are evaluated in 29 patients with psoriatic arthritis who have failed or have developed intolerable side-effects to non-steroidal antiinflammatory agents. In this randomized, double-blind, placebo-controlled, escalating, multiple dose trial, patients receive a 28 day course of daily subcutaneous IL-10 (1.0, 5.0, 10.0 or 20.0 microgram/kg) or placebo. At the end of the study, patients receiving placebo have the option to receive IL-10 (20 microgram/kg). Disease activity (both skin and joints) and toxicity are being monitored throughout the study. Research studies include functional assays of peripheral blood and synovial fluid mononuclear cells; cytokine gene expression (blood, synovium, skin); and activation of transcription factors specific for IL-10. This study examines the clinical and biologic effects of IL-10, a cytokine which is intimately involved in the regulation of immune and inflammatory responses. Psoriatic arthritis results in significant decline of function of affected patients and considerable social and financial costs. This study not only explores the potential clinical effects of IL-10, but also investigates in vivo the biological effects of prolonged exposure to this immunoregulatory cytokine.